Feel free to kill me.
1. An extreme (sarcastic) way of saying you give up or don't want to deal with a current situation.
2. A taint or an invitation to your own death (possibly), (or just an altercation that will leave the assailant wishing that they had been killed and not humiliated)
3. Used to express that you have acquired everything that you could ever want or desire.

Phill: I'm going to really mess you up for no reason.
Bruce: FFTKM (dramatic delay between the f and the t).

(upon reaching your new estate in your preferred method of transportation (license plate bearing the letters fftkm) you are greeted by a room full of beautiful men/women bearing your favorite substance(s))